1. Field of Invention
This invention pertains to the art of thermal barrier coating systems having enhanced resistance to spallation in components exposed to high temperatures, and more specifically to a segmented thermal barrier coating system resulting from modification of the substrate surface and/or modification of one or more layers of a thermal barrier coating.
2. Description of the Related Art
Thermal barrier coatings (TBC) are known in the art in order to protect components which are subjected to high temperature operations. Generally, the thermal barrier coating system includes a ceramic layer for thermal insulation of the component in combination with an additional layer called a bond coat located between the substrate and the ceramic layer. The bond coat typically serves to increase adherence of the ceramic layer on the metallic substrate and protect the underlying substrate from oxidation.
When the substrate is subjected to repeated heating/cooling cycles, thermally induced stresses and strains are produced and accumulate within the thermal barrier coating system. The most common mechanism of failure of the thermal barrier coating system is the spallation of the coating in local regions of the protected component. A crack is produced in the thermal barrier coating where it propagates until a portion of the coating system flakes or chips away. Such spallation failure may occur in patches or over an entire surface.
A number of techniques have been developed to reduce the tendency toward spallation failure of the thermal barrier coating. For example, U.S. Pat. No. 5,558,922 to Gupta discloses a thermal barrier coating for reducing long range stresses. Grooves are formed in the thermal barrier coating in a grid-like pattern, with a groove width of 0.1-0.5 mm (0.004-0.02 inches) and widely spaced from 10-250 mm (0.4-10 inches). This technique is usually directed for use with thick (750 micron, 0.03 inch) TBCs.
As disclosed in U.S. Pat. No. 5,840,434 to Kojima, a segmented thermal barrier coating is produced by separation between columns of the thermal barrier coating grains. This microstructure is produced by control of a physical vapor deposition (PVD) process. The induced cracks are very fine, from 5-10 microns, and closely spaced, about 100-200 microns.
U.S. Pat. No. 5,652,044 to Rickerby discloses a thermal barrier coating comprising a plurality of alternating layers to produce a plurality of interfaces substantially parallel to the metallic substrate/bond coating interface. The structure in the alternating layers is columnar, similar to the microstructure taught in the Kojima reference.
U.S. Pat. No. 5,419,971 to Skelly discloses a very fine surface texturing by means of a grid pattern of v-shaped grooves produced by laser grooving for the purpose of impeding crack growth at the interface of the thermal bond coat and the substrate. The surface features are only about 0.0005 inches deep by 0.0005 inches wide and spaced about 0.005 inches apart.
Other art relevant to the present invention includes U.S. Pat. No. 5,538,796 to Schaeffer et al. wherein an electron beam physical vapor deposition (EB-PVD) thermal barrier coating deposited on a low sulfur or hydrogen annealed single crystal superalloy is disclosed.
Additionally, U.S. Pat. No. 5,302,465 to Miller et al. describes a technique for depositing a yttria stabilized zirconia thermal barrier coating directly onto an oxidation resistant metal without a bond coat.
Although progress has been made, there still exists a need in the art to reduce spallation failure for components subjected to hostile environments.